But the fact is, "the body delivers only a certain percentage of vitamins to your skin, no matter how much you ingest," says Dr. Mary Lupo, clinical professor of dermatology at Tulane University School of Medicine. Plus, there's no way to send them straight to your crow's feet or brown spots. The solution: applying vitamins topically to deliver maximum benefits — everything from improving texture and tone to fading under-eye circles. Follow this user's guide to the letter, and soon your skin will look better than ever.

Vitamin A: best overall age fighterFind it in OTC lotions, night creams (vitamin A derivatives are known as retinoids), and prescription products.

How to use: Apply your retinoid at night — sunlight inactivates most forms of vitamin A. Prescription retinoids work fastest, within 4 to 8 weeks. The downside: They're irritating, causing redness, scaling, and flaking that last for weeks or longer. OTC products are best for beginners; you'll experience fewer side effects because the retinol they contain is slowly converted to retinoic acid, the active ingredient in Rx creams. To avoid irritation, apply an OTC or prescription retinoid every second or third night, at least for the first 2 weeks, and build up to nightly use. Apply sparingly; a pea-size amount is enough to cover your entire face.

Try: Neutrogena Dermatologics Retinol NX Serum ($65; qvc.com) or RoC Multi-Correxion Night Treatment ($25; drugstores). If your skin is sensitive, two new retinoids are particularly gentle. Clinical studies show that retinyl propionate, available in Olay Professional Pro-X Deep Wrinkle Treatment ($42; drugstores), significantly improves skin after 12 weeks without being as drying as the more potent retinol. Ready to trade up to an Rx retinoid? Ask about Atralin (about $150), which contains two potent emollients. "Even my most sensitive patients are able to tolerate it," says Day.

Vitamin B3: boosts hydration to reduce rednessFind it in lotions, creams, and serums. It's often called niacinamide on the label.

Proven to: increase production of ceramides and fatty acids, two key components of your skin's outer protective barrier. "As that barrier is strengthened, skin is better able to keep moisture in and irritants out — making B3 a great ingredient if your complexion is dry or sensitive," says Leslie S. Baumann, MD, director of the University of Miami Cosmetic Medicine and Research Institute. In one study, a moisturizer with niacinamide improved the flushing and blushing of rosacea, a common condition that can worsen with age. Another B3 benefit: It inhibits the transfer of pigment to skin cells, minimizing dark spots.

How to use: For maximum results, apply B3 in the morning and evening. To reduce irritation from your retinoid, use it in conjunction with niacinamide. "Mix them together in the palm of your hand before applying — they won't inactivate each other," says Baumann. Besides decreasing side effects, the combo produces superior anti-aging benefits.

Vitamin C: all-around anti-ager
Find it in moisturizers formulated to keep vitamin C stable (opaque, airtight containers are ideal). Look for C near the middle of the ingredients panel to help ensure the 5 percent or higher concentration needed to see benefits, advises Hema Sundaram, MD, a dermatologist in the Washington, DC, area.

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Proven to: mop up the free radicals that trigger wrinkling, sagging, and other aging changes. Vitamin C also helps smooth and firm skin and fade brown spots. In one study, women who treated sun-damaged skin with a C cream for 6 months saw significant improvement in fine lines and discoloration. Though the benefits of retinoids (see vitamin A) and vitamin C sound similar, using both delivers more complexion perfection. "Skin aging occurs in various ways, so you need multiple forms of defense and repair," says Lupo.

How to use: Apply vitamin C in the morning before sunscreen to shield your skin from any UV-generated free radicals that get by your sunblock.

Vitamin E: eases dryness and bolsters skin’s UV defense
Find it in sunscreens and after-sun products. The best products contain at least 1 percent vitamin E, so it will be listed near the middle of the ingredients panel.

Proven to: quell dryness by helping skin retain its natural moisturizers. Also, vitamin E's potent ability to neutralize damaging free radicals has earned it the moniker "the protector." A slew of studies document its superstar status. In one, E significantly reduced the number of these unstable molecules created after exposure to cigarette smoke. Others show that when it's used before UV exposure, skin is less red, swollen, and dry.

How to use: Apply before and after serious sun exposure. A single strong blast of UV light can destroy half the skin's natural supply of E, so shore up defenses by slathering on a sunscreen supplemented with E and C before going into the sun — the C helps ensure effectiveness. An after-sun salve with E helps, too, says Oceanside, CA, dermatologist Jens Thiele, MD, PhD, a vitamin E expert; some studies show that the anti-inflammatory action kicks in to reduce damage even after you've been in the sun.

Vitamin K: for younger, brighter eyes
Find it in eye creams that also contain retinol.

Proven to: possibly help lighten under-eye circles. Fragile capillaries that allow blood to leak into skin are considered one cause of under-eye circles, and vitamin K (aka phytonadione) may put the skids on this seepage by controlling blood clotting. Daily use of a K cream significantly lightened circles after 4 months in one study, but because the cream also contained retinol, researchers aren't sure which ingredient deserves credit for the improvement — retinol alone thickens the translucent under-eye skin (making it harder to see the dark blood vessels below) and lightens melanin that makes circles more prominent. Still, it can't hurt to try a cream that contains vitamin K and retinol; according to Baumann, the retinol may enhance K's ability to penetrate skin and knock out darkness.

How to use: Apply nightly. First allow skin to become acclimated to the retinol — use once or twice the first week, and add a night every week after.